Disk drives comprise a disk and a head connected to a distal end of an actuator arm which is rotated about a pivot by a voice coil motor (VCM) to position the head radially over the disk. The disk comprises a plurality of radially spaced, concentric tracks for recording user data sectors and servo sectors. The servo sectors comprise head positioning information (e.g., a track address) which is read by the head and processed by a servo control system to control the actuator arm as it seeks from track to track.
FIG. 1 shows a prior art disk format 2 as comprising a number of servo tracks 4 defined by servo sectors 60-6N recorded around the circumference of each servo track. Each servo sector 6i comprises a preamble 8 for storing a periodic pattern, which allows proper gain adjustment and timing synchronization of the read signal, and a sync mark 10 for storing a special pattern used to symbol synchronize to a servo data field 12. The servo data field 12 stores coarse head positioning information, such as a servo track address, used to position the head over a target data track during a seek operation. Each servo sector 6i further comprises groups of servo bursts 14 (A, B, C, D in the example shown), which are recorded with precise intervals and offsets relative to the servo track centerlines. The servo bursts 14 provide fine head position information used for centerline tracking while accessing a data track during write/read operations.
It is desirable to accurately predict when components in the disk drive might be degrading or failing, such as the read element, or the write element, or the laser used to heat the disk surface with heat assisted magnetic recording (HAMR). When a failure is predicted, the disk drive may take appropriate steps, such as recalibrating a bias current applied to the read element, recalibrating a write current applied to the write element, or recalibrating the laser power for HAMR. Predicting degradation or failure of the disk drive may trigger remedial measures to protect against data loss, such as performing write verify operations or increasing the frequency of data refresh. In other cases, the end user may be notified with an error message when degradation or failure of the disk drive is predicted.